Pressure waves have been used for many biological applications including the destruction of kidney stones (lithotripsy), the delivery of drug compounds and imaging particles to cells and tissues, gene therapy, cancer treatment, insulin and allergen delivery, and the modification of tissues. Many of these biological applications were achieved due to the ability of the energy carried by pressure waves to permeabilize the epidermis, cell plasma membranes, and cell nuclear envelopes, thereby creating a temporary pathway through which the desired molecules were able to penetrate these barriers.
Existing devices generate pressure waves using energy sources such as laser pulses, rapid diaphragm releases, compressed gas, electrostatic discharges, or by initiation of micro-explosions. The characteristics of the pressure waves differ from device to device, and are critical to the effectiveness of these devices in biological applications.
Shock waves are pressure waves that are generated by a source moving faster than the speed of sound in the medium carrying the shock waves. The effects of the forces induced by the impingement of shock waves on cells and tissues depend on the characteristics of the shock waves, including rise time, peak pressure, and pressure impulse. For example, shock waves possessing short rise times, high peak pressures, or high pressure impulses have been correlated with a high degree of cell permeabilization.
However, shock waves may cause cell injury or destruction if the peak pressure or the pressure impulse exceeds a threshold level. Although the threshold for cell injury appears to be higher than the threshold for cell permeabilization, the thresholds for permeabilization and cell injury vary as a function of the particular cell line and molecules being introduced to the cells. For biological applications, the ability to fine-tune the characteristics of the shock waves to fall between the threshold of permeabilization and the threshold for cell damage would enhance the utility of a device using shock waves to deliver compounds to cells.
Existing devices are relatively large in size and the controllability of the shock wave parameters such as peak pressure, pressure impulse and rise time are limited. Laser-generated shock waves possess high peak pressures, but the pressure impulses of the shock waves are limited due to the durations of these shock waves, which are on the order of nanoseconds. Gas-generated shock waves have a longer duration in the range of microseconds, but the peak pressure is relatively limited. Shock waves may also be created using the detonation of energetic materials such as lead azide. However, heavy-metal azides are sensitive to accidental detonation, and lead-based materials are environmentally hazardous, severely limiting the utility of heavy-metal azide detonation as a technique of generating pressure waves in biological applications.
A need exists for a suitably small device that generates shock waves with high peak pressures, fast rise times, and high pressure impulses, with the added ability to fine-tune the characteristics of the shock waves to match the requirements of individual cell types and compounds to be delivered.